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II. Mendelism and the Chromosomal Theory

II. Mendelism and the Chromosomal Theory

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Tamarin: Principles of

Genetics, Seventh Edition



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

Companies, 2001



Mendel’s Experiments



enetics is concerned with the transmission,

expression, and evolution of genes, the molecules that control the function, development, and ultimate appearance of individuals. In this section of the book, we will look

at the rules of transmission that govern genes and affect

their passage from one generation to the next. Gregor

Johann Mendel discovered these rules of inheritance; we

derive and expand upon his rules in this chapter (fig. 2.1).

In 1900, three botanists, Carl Correns of Germany,

Erich von Tschermak of Austria, and Hugo de Vries of

Holland, defined the rules governing the transmission of

traits from parent to offspring. Some historical controversy exists as to whether these botanists actually rediscovered Mendel’s rules by their own research or whether

their research led them to Mendel’s original paper. In any

case, all three made important contributions to the early

stages of genetics. The rules had been published previously, in 1866, by an obscure Austrian monk, Gregor Johann Mendel. Although his work was widely available after 1866, the scientific community was not ready to

appreciate Mendel’s great contribution until the turn of

the century. There are at least four reasons for this lapse

of thirty-four years.



G



17



First, before Mendel’s experiments, biologists were

primarily concerned with explaining the transmission of

characteristics that could be measured on a continuous

scale, such as height, cranium size, and longevity. They

were looking for rules of inheritance that would explain

such continuous variations, especially after Darwin

put forth his theory of evolution in 1859 (see chapter 21). Mendel, however, suggested that inherited characteristics were discrete and constant (discontinuous):

peas, for example, were either yellow or green.Thus, evolutionists were looking for small changes in traits with

continuous variation, whereas Mendel presented them

with rules for discontinuous variation. His principles did

not seem to apply to the type of variation that biologists

thought prevailed. Second, there was no physical element identified with Mendel’s inherited entities. One

could not say, upon reading Mendel’s work, that a certain

subunit of the cell followed Mendel’s rules.Third, Mendel

worked with large numbers of offspring and converted

these numbers to ratios. Biologists, practitioners of a very

descriptive science at the time, were not well trained in

mathematical tools. And last, Mendel was not well known

and did not persevere in his attempts to convince the academic community that his findings were important.

Between 1866 and 1900, two major changes took

place in biological science. First, by the turn of the century, not only had scientists discovered chromosomes,

but they also had learned to understand chromosomal

movement during cell division. Second, biologists were

better prepared to handle mathematics by the turn of the

century than they were during Mendel’s time.



MENDEL’S EXPERIMENTS



Figure 2.1



Gregor Johann Mendel (1822–84).



permission of the Moravski Museum, Mendelianum.)



(Reproduced by



Gregor Mendel was an Austrian monk (of Brünn, Austria,

which is now Brno, Czech Republic). In his experiments,

he tried to crossbreed plants that had discrete, nonoverlapping characteristics and then to observe the distribution of these characteristics over the next several generations. Mendel worked with the common garden pea

plant, Pisum sativum. He chose the pea plant for at least

three reasons: (1) The garden pea was easy to cultivate

and had a relatively short life cycle. (2) The plant had discontinuous characteristics such as flower color and pea

texture. (3) In part because of its anatomy, pollination of

the plant was easy to control. Foreign pollen could be

kept out, and cross-fertilization could be accomplished artificially.

Figure 2.2 shows a cross section of the pea flower

that indicates the keel, in which the male and female

parts develop. Normally, self-fertilization occurs when

pollen falls onto the stigma before the bud opens.

Mendel cross-fertilized the plants by opening the keel of



Tamarin: Principles of

Genetics, Seventh Edition



18



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

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Chapter Two Mendel’s Principles



Filament

Stigma

Anther

Style

Ovary

Keel

(half cut

away)



Anatomy of the garden pea plant flower. The female

part, the pistil, is composed of the stigma, its supporting style,

and the ovary. The male part, the stamen, is composed of the

pollen-producing anther and its supporting filament.



ent forms of a gene that exist within a population are

termed alleles. The terms dominant and recessive are

used to describe both the relationship between the alleles and the traits they control. Thus, we say that both

the allele for tallness and the trait, tall, are dominant.

Dominance applies to the appearance of the trait when

both a dominant and a recessive allele are present. It

does not imply that the dominant trait is better, is more

abundant, or will increase over time in a population.

When the F1 offspring of figure 2.4 were selffertilized to produce the F2 generation, both tall and

dwarf offspring occurred; the dwarf characteristic reappeared. Among the F2 offspring, Mendel observed 787

tall and 277 dwarf plants for a ratio of 2.84:1. It is an indication of Mendel’s insight that he recognized in these

numbers an approximation to a 3:1 ratio, a ratio that suggested to him the mechanism of inheritance at work in

pea plant height.



Figure 2.2



a flower before the anthers matured and placing pollen

from another plant on the stigma. In the more than ten

thousand plants Mendel examined, only a few were fertilized other than the way he had intended (either self- or

cross-pollinated).

Mendel used plants obtained from suppliers and

grew them for two years to ascertain that they were homogeneous, or true-breeding, for the particular characteristic under study. He chose for study the seven characteristics shown in figure 2.3. Take as an example the

characteristic of plant height. Although height is often

continuously distributed, Mendel used plants that displayed only two alternatives: tall or dwarf. He made the

crosses shown in figure 2.4. In the parental, or P1, generation, dwarf plants pollinated tall plants, and, in a reciprocal cross, tall plants pollinated dwarf plants, to determine whether the results were independent of the

parents’ sex. As we will see later on, some traits follow inheritance patterns related to the sex of the parent carrying the traits. In those cases, reciprocal crosses give different results; with Mendel’s tall and dwarf pea plants,

the results were the same.

Offspring of the cross of P1 individuals are referred to

as the first filial generation, or F1. Mendel also referred

to them as hybrids because they were the offspring of

unlike parents (tall and dwarf). We will specifically refer

to the offspring of tall and dwarf peas as monohybrids

because they are hybrid for only one characteristic

(height). Since all the F1 offspring plants were tall,

Mendel referred to tallness as the dominant trait. The alternative, dwarfness, he referred to as recessive. Differ-



S E G R E G AT I O N

Rule of Segregation

Mendel assumed that each plant contained two determinants (which we now call genes) for the characteristic

of height. For example, a hybrid F1 pea plant possesses

the dominant allele for tallness and the recessive allele

for dwarfness for the gene that determines plant height.

A pair of alleles for dwarfness is required to develop the

recessive phenotype. Only one of these alleles is passed

into a single gamete, and the union of two gametes to

form a zygote restores the double complement of alleles.

The fact that the recessive trait reappears in the F2 generation shows that the allele controlling it was hidden in

the F1 individual and passed on unaffected. This explanation of the passage of discrete trait determinants, or

genes, comprises Mendel’s first principle, the rule of

segregation. The rule of segregation can be summarized

as follows: A gamete receives only one allele from the

pair of alleles an organism possesses; fertilization (the

union of two gametes) reestablishes the double number.

We can visualize this process by redrawing figure 2.4 using letters to denote the alleles. Mendel used capital letters to denote alleles that control dominant traits and

lowercase letters for alleles that control recessive traits.

Following this notation, T refers to the allele controlling

tallness and t refers to the allele controlling shortness

(dwarf stature). From figure 2.5, we can see that Mendel’s

rule of segregation explains the homogeneity of the F1

generation (all tall) and the 3:1 ratio of tall-to-dwarf offspring in the F2 generation.

Let us define some terms. The genotype of an organism is the gene combination it possesses. In figure 2.5,



Tamarin: Principles of

Genetics, Seventh Edition



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

Companies, 2001



Segregation



Alternative forms



Seeds



Pods



Stem



(1)



Round



Wrinkled



(2)



Yellow

cotyledons



Green

cotyledons



(3)



Gray coat

(violet flowers)



White coat

(white flowers)



(4)



Full



Constricted



(5)



Green



Yellow



(6)



Axial pods

and flowers

along stem



Terminal pods

and flowers on

top of stem



(7)



Tall

(6–7 ft)



Dwarf

(3/4–1 ft)



Seven characteristics that Mendel observed in peas. Traits in the left column

are dominant.



Figure 2.3



19



Tamarin: Principles of

Genetics, Seventh Edition



20



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

Companies, 2001



Chapter Two Mendel’s Principles



P1



×



Tall



Dwarf



F1



× Self



Tall



F2



Tall



Dwarf

3 : 1



Figure 2.4 First two offspring generations from the cross of tall plants with dwarf plants.



Tamarin: Principles of

Genetics, Seventh Edition



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

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21



Segregation



TT

Tall



P1



×



tt

Dwarf



Schematic



Tt

Gametes



t



T



Tt

Tall



F1



Gametes



T



or



t



Pollen

Tt



× Self



Ovule

Tt



×



Tt



Tt



T



or



Tt



X



(as in fig. 2.5)



T

+



T

+



t

+



t

+



T



t



T



t



TT

1

t



:



Tt

2



:



tt

1



Diagrammatic

(Punnett square)



TT



Tt



Tt



Pollen



tt

1/4



3/4

Tall



Ovules



F2



Dwarf

3:1



Figure 2.5



T



t



T



TT



Tt



t



Tt



tt



TT Tt tt

1 : 2 : 1



Assigning genotypes to the cross in figure 2.4.

Probabilistic



the genotype of the parental tall plant is TT; that of the F1

tall plant is Tt. Phenotype refers to the observable attributes of an organism. Plants with either of the two

genotypes T T or Tt are phenotypically tall. Genotypes

come in two general classes: homozygotes, in which

both alleles are the same, as in TT or tt, and heterozygotes, in which the two alleles are different, as in Tt.

William Bateson coined these last two terms in 1901.

Danish botanist Wilhelm Johannsen first used the word

gene in 1909.

If we look at figure 2.5, we can see that the T T

homozygote can produce only one type of gamete, the

T-bearing kind, and the tt homozygote can similarly produce only t-bearing gametes. Thus, the F1 individuals are

uniformly heterozygous Tt, and each F1 individual can

produce two kinds of gametes in equal frequencies, T- or

t-bearing. In the F2 generation, these two types of gametes randomly pair during fertilization. Figure 2.6

shows three ways of picturing this process.



Testing the Rule of Segregation

We can see from figure 2.6 that the F2 generation has a

phenotypic ratio of 3:1, the classic Mendelian ratio.

However, we also see a genotypic ratio of 1:2:1 for dominant homozygote:heterozygote:recessive homozygote.

Demonstrating this genotypic ratio provides a good test

of Mendel’s rule of segregation.

The simplest way to test the hypothesis is by progeny testing, that is, by self-fertilizing F2 individuals to



(Multiply; see rule 2, chapter 4.)

Pollen



Ovules



1/2



T



=



1/4 TT



1/2



t



=



1/4 Tt



1



1/2 T



2



1/2



1/2



T



=



1/4 Tt



1/2



t



=



1/4 tt



t

1



Figure 2.6 Methods of determining F2 genotypic combinations

in a self-fertilized monohybrid. The Punnett square diagram is

named after the geneticist Reginald C. Punnett.



produce an F3 generation, which Mendel did (fig. 2.7).

Treating the rule of segregation as a hypothesis, it is possible to predict the frequencies of the phenotypic classes

that would result. The dwarf F2 plants should be recessive homozygotes, and so, when selfed (self-fertilized),

they should produce only t-bearing gametes and only

dwarf offspring in the F3 generation. The tall F2 plants,

however, should be a heterogeneous group, one-third of

which should be homozygous T T and two-thirds heterozygous Tt. The tall homozygotes, when selfed, should

produce only tall F3 offspring (genotypically TT ). However, the F2 heterozygotes, when selfed, should produce



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II. Mendelism and the

Chromosomal Theory



2. Mendels Principles



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Chapter Two Mendels Principles



Tall

F2



F3



Dwarf



ì Self



TT



Tall

100%



Tt



Tall



ì Self



tt



Dwarf

3 : 1



× Self



Dwarf

100%



Figure 2.7 Mendel self-fertilized F2 tall and dwarf plants. He found that

all the dwarf plants produced only dwarf progeny. Among the tall plants,

72% produced both tall and dwarf progeny in a 3:1 ratio.



Genotype to be tested



Gamete



AA



Gamete



Aa



×



Gamete of aa



A



×



a



=



=



Aa

(dominant phenotype)



=



Aa

(dominant phenotype)

aa

(recessive phenotype)



A



×

a



a



Offspring



Testcross. In a testcross, the phenotype of an offspring is

determined by the allele the offspring inherits from the parent with the

genotype being tested.



Figure 2.8



tall and dwarf offspring in a ratio identical to that the

selfed F1 plants produced: three tall to one dwarf offspring. Mendel found that all the dwarf (homozygous) F2

plants bred true as predicted. Among the tall, 28%

(28/100) bred true (produced only tall offspring) and

72% (72/100) produced both tall and dwarf offspring.

Since the prediction was one-third (33.3%) and twothirds (66.7%), respectively, Mendel’s observed values

were very close to those predicted. We thus conclude

that Mendel’s progeny-testing experiment confirmed his

hypothesis of segregation. In fact, a statistical test—

developed in chapter 4—would also the support this

conclusion.

Another way to test the segregation rule is to use the

extremely useful method of the testcross, that is, a cross

of any organism with a recessive homozygote. (Another

type of cross, a backcross, is the cross of a progeny with

Tall (two classes)



TT

Tt



× tt = all Tt

× tt =Tt : tt

1:1



Testcrossing the dominant phenotype of the F2

generation from figure 2.5.

Figure 2.9



an individual that has a parental genotype. Hence, a testcross can often be a backcross.) Since the gametes of the

recessive homozygote contain only recessive alleles, the

alleles that the gametes of the other parent carry will determine the phenotypes of the offspring. If a gamete

from the organism being tested contains a recessive allele, the resulting F1 organism will have a recessive phenotype; if it contains a dominant allele, the F1 organism

will have a dominant phenotype. Thus, in a testcross, the

genotypes of the gametes from the organism being

tested determine the phenotypes of the offspring

(fig. 2.8). A testcross of the tall F2 plants in figure 2.5

would produce the results shown in figure 2.9. These results further confirm Mendel’s rule of segregation.



DOMINANCE IS NOT

UNIVERSAL

If dominance were universal, the heterozygote would always have the same phenotype as the dominant homozygote, and we would always see the 3:1 ratio when

heterozygotes are crossed. If, however, the heterozygote

were distinctly different from both homozygotes, we



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Chromosomal Theory



2. Mendels Principles



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Nomenclature



P1



White

R2R2



ì



Red

R1R1

F1



Pink

R1R2



ì



Self



F2



Red

R1R1



Pink

R1R2



White

R2R2



1:2:1



Flower color inheritance in the four-o’clock plant:

an example of partial, or incomplete, dominance.



Figure 2.10



would see a 1:2:1 ratio of phenotypes when heterozygotes are crossed. In partial dominance (or incomplete dominance), the phenotype of the heterozygote

falls between those of the two homozygotes. An example

occurs in flower petal color in some plants.

Using four-o’clock plants (Mirabilis jalapa), we can

cross a plant that has red flower petals with another that

has white flower petals; the offspring will have pink

flower petals. If these pink-flowered F1 plants are

crossed, the F2 plants appear in a ratio of 1:2:1, having

red, pink, or white flower petals, respectively (fig. 2.10).

The pink-flowered plants are heterozygotes that have a

petal color intermediate between the red and white colors of the homozygotes. In this case, one allele (R1) specifies red pigment color, and another allele specifies no

color (R2; the flower petals have a white background



23



color). Flowers in heterozygotes (R1R2) have about half

the red pigment of the flowers in red homozygotes

(R1R1) because the heterozygotes have only one copy of

the allele that produces color, whereas the homozygotes

have two copies.

As technology has improved, we have found more

and more cases in which we can differentiate the heterozygote. It is now clear that dominance and recessiveness are phenomena dependent on which alleles are interacting and on what phenotypic level we are studying.

For example, in Tay-Sachs disease, homozygous recessive

children usually die before the age of three after suffering

severe nervous system degeneration; heterozygotes seem

to be normal. As biologists have discovered how the disease works, they have made the detection of the heterozygotes possible.

As with many genetic diseases, the culprit is a defective enzyme (protein catalyst). Afflicted homozygotes

have no enzyme activity, heterozygotes have about half

the normal level, and, of course, homozygous normal individuals have the full level. In the case of Tay-Sachs disease, the defective enzyme is hexoseaminidase-A, needed

for proper lipid metabolism. Modern techniques allow

technicians to assay the blood for this enzyme and to

identify heterozygotes by their intermediate level of enzyme activity. Two heterozygotes can now know that

there is a 25% chance that any child they bear will have

the disease. They can make an educated decision as to

whether or not to have children.

The other category in which the heterozygote is discernible occurs when the heterozygous phenotype is

not on a scale somewhere between the two homozygotes, but actually expresses both phenotypes simultaneously. We refer to this situation as codominance. For

example, people with blood type AB are heterozygotes

who express both the A and B alleles for blood type (see

the section entitled “Multiple Alleles” for more information about blood types). Electrophoresis (a technique described in chapter 5) lets us see proteins directly and also

gives us many examples of codominance when we can

see the protein products of both alleles.



N O M E N C L AT U R E

Throughout the last century, botanists, zoologists, and

microbiologists have adopted different methods for naming alleles. Botanists and mammalian geneticists tend to

prefer the capital-lowercase scheme. Drosophila geneticists and microbiologists have adopted schemes that relate to the wild-type. The wild-type is the phenotype of

the organism commonly found in nature. Though other

naturally occurring phenotypes of the same species may

also be present, there is usually an agreed-upon common



Tamarin: Principles of

Genetics, Seventh Edition



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II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

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Chapter Two Mendel’s Principles



Table 2.1 Some Mutants of Drosophila

Mutant

Designation



Adult male

Figure 2.11



Adult female



Description



Dominance

Relationship

to Wild-Type



abrupt (ab)



Shortened, longitudinal,

median wing vein



Recessive



amber (amb)



Pale yellow body



Recessive



black (b)



Black body



Recessive



Bar (B)



Narrow, vertical eye



Dominant



dumpy (dp)



Reduced wings



Recessive



Hairless (H )



Various bristles absent



Dominant



white (w)



White eye



Recessive



white-apricot

(w a)



Apricot-colored eye

(allele of white eye)



Recessive



Wild-type fruit fly, Drosophila melanogaster.



phenotype that is referred to as the wild-type. For fruit

flies (Drosophila), organisms commonly used in genetic

studies, the wild-type has red eyes and round wings

(fig. 2.11). Alternatives to the wild-type are referred to as

mutants (fig. 2.12). Thus, red eyes are wild-type, and

white eyes are mutant. Fruit fly genes are named after the

mutant, beginning with a capital letter if the mutation is

dominant and a lowercase letter if it is recessive.

Table 2.1 gives some examples.The wild-type allele often

carries the symbol of the mutant with a ϩ added as a



Cy



sd



dp



superscript; by definition, every mutant has a wild-type

allele as an alternative. For example, w stands for the

white-eye allele, a recessive mutation. The wild-type (red

eyes) is thus assigned the symbol wϩ. Hairless is a dominant allele with the symbol H. Its wild-type allele is denoted as Hϩ. Sometimes geneticists use the ϩ symbol

alone for the wild-type, but only when there will be no

confusion about its use. If we are discussing eye color

only, then ϩ is clearly the same as wϩ: both mean red

eyes. However, if we are discussing both eye color and

bristle morphology, the ϩ alone could refer to either of

the two aspects of the phenotype and should be avoided.



ap



D



vg



c



Wing mutants of Drosophila melanogaster and their allelic designations: Cy, curly; sd, scalloped; ap, apterous; vg,

vestigial; dp, dumpy; D, Dichaete; c, curved.



Figure 2.12



Tamarin: Principles of

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II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



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25



Multiple Alleles



Table 2.2 ABO Blood Types with Immunity Reactions

Blood Type Corresponding

to Antigens on Red

Blood Cells



Antibodies in Serum



Genotype



Reaction of Red

Cells to Anti-A

Antibodies



Reaction of Red

Cells to Anti-B

Antibodies

Ϫ



O



Anti-A and anti-B



ii



Ϫ



A



Anti-B



IAIA or IAi



ϩ



Ϫ



B



Anti-A



IBIB or IBi



Ϫ



ϩ



AB



None



IAIB



ϩ



ϩ



other antigen: type A persons have A antigen on their red

cells and anti-B antibody in their serum; type B persons

have B antigen on their red cells and anti-A antibody in

their serum; type O persons do not have either antigen

but have both antibodies in their serum; and type AB

persons have both A and B antigens and form neither

anti-A or anti-B antibodies in their serum.

The I A and I B alleles, coding for glycosyl transferase

enzymes, each cause a different modification to the terminal sugars of a mucopolysaccharide (H structure)

found on the surface of red blood cells (fig. 2.13). They

are codominant because both modifications (antigens)

are present in a heterozygote. In fact, whichever enzyme

(product of the I A or I B allele) reaches the H structure

first will modify it. Once modified, the H structure will

not respond to the other enzyme. Therefore, both A and

B antigens will be produced in the heterozygote in

roughly equal proportions. The i allele causes no change

to the H structure: because of a mutation it produces a

nonfunctioning enzyme. The i allele and its phenotype

are recessive; the presence of the I A or I B allele, or both,



MULTIPLE ALLELES

A given gene can have more than two alleles. Although

any particular individual can have only two, many alleles

of a given gene may exist in a population. The classic example of multiple human alleles is in the ABO blood

group, which Karl Landsteiner discovered in 1900.This is

the best known of all the red-cell antigen systems primarily because of its importance in blood transfusions.

There are four blood-type phenotypes produced by three

alleles (table 2.2).The I A and I B alleles are responsible for

the production of the A and B antigens found on the surface of the erythrocytes (red blood cells). Antigens are

substances, normally foreign to the body, that induce the

immune system to produce antibodies (proteins that

bind to the antigens).The ABO system is unusual because

antibodies can be present (e.g., anti-B antibodies can exist in a type A person) without prior exposure to the antigen. Thus, people with a particular ABO antigen on their

red cells will have in their serum the antibody against the



H structure



Fucose



Gal



I A allele

(Galnac added

to H structure)



i allele

(no change in

H structure)



Fucose



Gal



Glunac



Glunac



Fucose



Gal



Glunac



Galnac



I B allele

(Gal added to

H structure)



Fucose



Gal



Glunac



Gal



Gal

= Galactose

Galnac = N-Acetylgalactosamine

Glunac = N-Acetylglucosamine



Function of the IA, IB, and i alleles of the ABO gene. The gene products of the IA and IB alleles of the ABO gene affect

the terminal sugars of a mucopolysaccharide (H structure) found on red blood cells. The gene products of the IA and IB alleles are

the enzymes alpha-3-N-acetyl-D-galactosaminyltransferase and alpha-3-D-galactosyltransferase, respectively.

Figure 2.13



Tamarin: Principles of

Genetics, Seventh Edition



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II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

Companies, 2001



Chapter Two Mendel’s Principles



will modify the H product, thus masking the fact that

the i allele was ever there.

Adverse reactions to blood transfusions primarily occur

because the antibodies in the recipient’s serum react with

the antigens on the donor’s red blood cells. Thus, type A

persons cannot donate blood to type B persons. Type B

persons have anti-A antibody, which reacts with the A antigen on the donor red cells and causes the cells to clump.

Since both I A and I B are dominant to the i allele, this

system not only shows multiple allelism, it also demonstrates both codominance and simple dominance. (As

with virtually any system, intense study yields more information, and subgroups of type A are known. We will

not, however, deal with that complexity here.) According

to the American Red Cross, 46% of blood donors in the

United States are type O, 40% are type A, 10% are type B,

and 4% are type AB.

Many other genes also have multiple alleles. In some

plants, such as red clover, there is a gene, the S gene, with

several hundred alleles that prevent self-fertilization. This

means that a pollen grain is not capable of forming a successful pollen tube in the style if the pollen grain or its

parent plant has a self-incompatibility allele that is also

present in the plant to be fertilized. Thus, pollen grains

from a flower falling on its own stigma are rejected. Only

a pollen grain with either a different self-incompatibility

allele or from a parent plant with different selfincompatibility alleles is capable of fertilization; this

avoids inbreeding. Thus, over evolutionary time, there

has been selection for many alleles of this gene. Presumably, a foreign plant would not want to be mistaken for

the same plant, providing the selective pressure for many

alleles to survive in a population. Recent research has indicated that the products of the S alleles are ribonuclease

enzymes, enzymes that destroy RNA. Researchers are interested in discovering the molecular mechanisms for

this pollen rejection.

In Drosophila, numerous alleles of the white-eye gene

exist, and people have numerous hemoglobin alleles. In

fact, multiple alleles are the rule rather than the exception.



an F2 generation that had all four possible combinations

of the two seed characteristics: round, yellow seeds;

round, green seeds; wrinkled, yellow seeds; and wrinkled, green seeds.The numbers Mendel reported in these

categories were 315, 108, 101, and 32, respectively. Dividing each number by 32 gives a 9.84 to 3.38 to 3.16 to

1.00 ratio, which is very close to a 9:3:3:1 ratio. As you

will see, this is the ratio we would expect if the genes

governing these two traits behaved independently of

each other.

In figure 2.14, the letter R is assigned to the dominant

allele, round, and r to the recessive allele, wrinkled; Y and

y are used for yellow and green color, respectively. In figure 2.15, we have rediagrammed the cross in figure 2.14.

The P1 plants in this cross produce only one type of gamete each, RY for the parent with the dominant traits

and ry for the parent with the recessive traits. The resulting F1 plants are heterozygous for both genes (dihybrid). Self-fertilizing the dihybrid (RrYy) produces the

F2 generation.

In constructing the Punnett square in figure 2.15 to

diagram the F2 generation, we make a critical assumption: The four types of gametes from each parent will be

produced in equal numbers, and hence every offspring

category, or “box,” in the square is equally likely.Thus, because sixteen boxes make up the Punnett square (named

after its inventor, Reginald C. Punnett), the ratio of F2 offspring should be in sixteenths. Grouping the F2 offspring

by phenotype, we find there are 9/16 that have round,

yellow seeds; 3/16 that have round, green seeds; 3/16

that have wrinkled, yellow seeds; and 1/16 that have

wrinkled, green seeds. This is the origin of the expected

9:3:3:1 F2 ratio.



INDEPENDENT ASSORTMENT

Mendel also analyzed the inheritance pattern of traits observed two at a time. He looked, for instance, at plants

that differed in the form and color of their peas: he

crossed true-breeding (homozygous) plants that had

seeds that were round and yellow with plants that produced seeds that were wrinkled and green. Mendel’s results appear in figure 2.14. The F1 plants all had round,

yellow seeds, which demonstrated that round was dominant to wrinkled and yellow was dominant to green.

When these F1 plants were self-fertilized, they produced



Reginald C. Punnett (1875–1967).

From Genetics, 58 (1968): frontispiece.

Courtesy of the Genetics Society of

America.



Tamarin: Principles of

Genetics, Seventh Edition



II. Mendelism and the

Chromosomal Theory



2. Mendel’s Principles



© The McGraw−Hill

Companies, 2001



27



Independent Assortment



P1



ϫ



Round, yellow



Wrinkled, green



RRYY



rryy



RY



ry



Gametes



P1

RrYy

F1



Round, yellow

(RRYY )



X



Wrinkled, green

(rryy )



RY



Gametes



rY



Ry



ry



1 : 1 : 1 : 1



Pollen



F1

X Self



F2



Ovules



Round, yellow

(RrYy )



F2



Round, yellow

(315)

(RRYY; RRYy;

RrYY; RrYy )



Round, green

(108)

(RRyy; Rryy )



Figure 2.15



Wrinkled, yellow

(101)

(rrYY; rrYy )

Figure 2.14



RY



Ry



rY



ry



RY



RRYY



RRYy



RrYY



RrYy



Ry



RRYy



RRyy



RrYy



Rryy



rY



RrYY



RrYy



rrYY



rrYy



ry



RrYy



Rryy



rrYy



rryy



Assigning genotypes to the cross in figure 2.14.



Wrinkled, green

(32)

(rryy )



Independent assortment in garden peas.



Rule of Independent Assortment

This ratio comes about because the two characteristics

behave independently. The F1 plants produce four types

of gametes (check fig. 2.15): RY, Ry, rY, and ry. These gametes occur in equal frequencies. Regardless of which

seed shape allele a gamete ends up with, it has a 50:50



chance of getting either of the alleles for color—the two

genes are segregating, or assorting, independently. This is

the essence of Mendel’s second rule, the rule of independent assortment, which states that alleles for one

gene can segregate independently of alleles for other

genes. Are the alleles for the two characteristics of color

and form segregating properly according to Mendel’s

first principle?

If we look only at seed shape (see fig. 2.14), we find

that a homozygote with round seeds was crossed with a

homozygote with wrinkled seeds in the P1 generation

(RR ϫ rr). This cross yields only heterozygous plants

with round seeds (Rr) in the F1 generation. When these



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